The goal of our research is to determine general and specific aspects of the structure and mechanism of the two isoenzymes of serine transhydroxymethylase. As steps toward this goal we propose the following three studies. (1) Determine the primary structure of both cytoplasmic and mitochondrial serine transhydroxymethylase. This study should help us understand the structural and functional relationship of these two forms of the enzyme. (2) Determine the role of the phosphate group of the coenzyme pyridoxal-5-phosphate in the mechanism of the reaction. This will be done by observing the 31P nmr signal of the enzyme bound coenzyme and several coenzyme analogs. (3) Determine the catalytic viability of the enzyme in several crystalline forms. This will be done by observing the formation of enzyme substrate complexes and the catalytic activity of the enzyme in single crystals. This study will add to our knowledge in general about the viability of enzymes in the crystalline state and in particular whether our future three dimensional studies are dealing with crystals in structurally competent form.